Cobalt-boron and nickel-boron plating baths have been in use since Talivaldis Berzins filed the first U.S. patent application on this type of process on Oct. 1, 1958, which matured into U.S. Pat. No. 3,096,182, issued Jul. 2, 1963. Conventional plating baths are generally used for making depositions on iron and iron alloy substrates. Most of these plating baths use sodium borohydride (NaBH4) as the source of boron. The Berzins patent and others typically recite process temperatures of from 60° C. to 100° C., at pH's of about 12 to 13 or higher. Baths of this type deposit nickel-boron or cobalt-boron alloys with a boron content of 3.5% to 6.0%. As a result, the coatings are very hard. They are used extensively on gun barrels and in many applications requiring a hard, scratch-resistant surface. Since the Berzins patent, a number of improvements have been patented, but there has been a strict adherence to maintaining the bath at a pH of from 12 to 14, and at temperatures of no less than 60° C. These conventional reaction conditions are echoed in the standard text on electroless nickel deposition techniques, “Electroless Plating Fundamentals and Applications,” G.O. Mallory and J. B. Hajdu, eds., ©1991 William Andrew Press, ISBN 978-0815512776. The high pH is required to prevent the decomposition of the borohydride. The conventional elevated temperature range is required to achieve an acceptably fast deposition rate. Iron and its alloys exhibit passivity at a high pH which counteracts the deposition. Additionally, absent adding catalysis to the bath (such as thallium and/or organic sulfur compounds) the maximum observed deposition rate is only about 0.0001 inch (2.54 μm) per hour at over 90° C. (See Mallory & Hajdu, supra, at page 82.). Thallium, however, is quite toxic and also plates out with the deposited nickel or cobalt. The co-deposited thallium imparts undesirable physical and chemical properties to the ultimate coating, as lower hardness, increased brittleness, and lower corrosion resistance.
To obtain a higher rate of deposition and a coating free of impurities introduced via a catalyst, amine-boranes came into use in the late 1960's. See U.S. Pat. No. 3,338,726, issued Aug. 29, 1967 to Berzins. The resulting deposited coatings contain less than 3.5% boron, deposit at higher rates, rate, may be used at temperatures as low as 40° C. and are conventionally used at pH's of about 6.0 to 9.5. The more mild deposition conditions also allow these types of baths to deposit onto aluminum and aluminum alloy substrates. They are used extensively in the electronics industry because the deposits may be soldered. However, due to the high cost of the amine-boranes, the baths are much more expensive to use. There are also adverse environmental issues associated with their use.